If the bottom is not painted, and you can get away with it (I can't for the barnacles would have a field day) then corrosion is less of a problem. Why? Because the AREA exposed to the water is so great. Paint the hull, then get a bare metal spot somewhere (from a scrape) and corrosion focuses on that area. The anodes prevent this by taking the hit themselves.
Hope that clarifies it.
Jeff
PS: Even with anti-fouling I have barnacles by the dozens to scrape every Fall.
"...reality is not nearly as lovely as the world of Liberal Land. No wonder so many people want to go there." - Tom Sowell